How soon should I remove a calf from the mother in order for the heifer to get pregnant?

I'm assuming you're referring to the calf as the heifer. The heifer won't be bred until she's at least 15 months old, though some folks like to give the heifer more time to grow before breeding her, such as not until she's almost 2 years old. A calf can stay on her mama for 6 months or up to 10 months old. When to wean depends on the cow, really, not so much the calf. A calf's most critical period for needing milk is up to 3 months old. A cow that is losing weight from producing milk for a growing calf older than 3 months is one that may need weaning. If that's not the case, then wean at any time you feel.